Nuke ShortStringField
Allow _get_value to accept raw default values
Remove code for unpacking server responses

Formerly, some server responses were in this format:

    {'entity_name': {...}}

And some server responses were in this format:

    {...}}

Bugzilla bug 1087372 deals with this issue. Remove the code that allowed the
test client to deal with either type of response, as only the latter type of
response is now expected.